Method for preparing an aqueous treatment solution containing at least hydrogen peroxide ions and hydroxyl ions in predetermined concentrations

ABSTRACT

The present invention provides a method for preparing an aqueous treatment solution containing at least hydrogen peroxide ions and hydroxyl ions in a predetermined concentration. 
     The method according to the invention consists of using an electrolytic cell comprising a porous cathode, and circulating simultaneously in contact with said porous cathode an oxygen-containing compressed gas and a weakly alkaline initial aqueous solution, so as to cause the formation of hydrogen peroxide ions and hydroxyl ions by reduction of the oxygen, said circulation being continued until said solution attains said predetermined concentration, said solution having attained said concentration then being discharged to the outside for use as the treatment solution in various applications. 
     Application in particular to bleaching, pollution control etc.

FIELD OF THE INVENTION

This invention relates to a method and reactor for preparing an aqueoustreatment solution containing at least hydrogen peroxide ions andhydroxyl ions in predetermined concentrations.

BACKGROUND OF THE INVENTION

Among known bleaching agents, hydrogen peroxide is at the present timebeing increasingly used, in particular for bleaching materials such astextiles or paper pulp. Hydrogen peroxide has the great advantage overother bleaching agents, in particular chlorine and its compounds, inthat because of its mild action, it attacks the fibers of the materialto be treated to a much lesser extent, while exerting a more durableaction and giving a better finish.

Hydrogen peroxide is generally used in bleaching in the form of astabilized alkaline solution of low peroxide concentration. The actionof the hydrogen peroxide in bleaching consists essentially of destroyingor decolourising the natural dyes by oxidation, or by rendering themsoluble. Even though the mechanism of these reactions has still beenlittle studied, it is generally assumed that the hydrogen peroxide ionHOO⁻ is responsible for the bleaching.

Present-day bleaching solutions based on hydrogen peroxide have howeverthe great disadvantage with respect to other conventional bleachingsolutions (in particular hypochlorite-based solutions) of beingrelatively costly, so that their widespread use is very dependent oneconomic considerations, in particular when large quantities oflow-value material such as paper pulp are to be treated. Present-daybleaching solutions are nearly always prepared by simple dissolving anddilution, starting from commercially available chemicals. Commerciallyavailable hydrogen peroxide is a particularly costly substance, as it ismanufactured only in a small number of large industrial plants, and ithas therefore to be highly concentrated for storage and transportpurposes before being distributed.

At the present time there is a need to replace these preparation methodsusing highly concentrated commercially available constituents, by insitu manufacturing methods which enable dilute solutions of hydrogenperoxide to be produced directly, in order to reduce bleaching costs.However, up to the present time no satisfactory method has appeared.

Hydrogen peroxide is used not only for bleaching purposes, but also inan increasing number of other processes, in particular in the pollutioncontrol field. However, treatment solutions used for this purpose arelikewise almost always prepared from highly concentrated commercialconstituents, and they thus have the same disadvantages as heretoforestated.

It has been proposed for some time to prepare hydrogen peroxideelectrolytically, by reducing oxygen at a cathode in an alkaline medium.However, the methods proposed up to the present time all aim atlarge-scale production of hydrogen peroxide, so that the productsobtained by these methods are much too concentrated for direct use astreatment solutions (the alkaline solutions used in these methods have avery high initial concentration).

OBJECT OF THE INVENTION

The object of the present invention is to obviate the aforesaiddisadvantages by proposing an electrolytic preparation process enablingdilute solutions of hydrogen peroxide to be produced in situ for directuse as treatment solutions in various applications.

SUMMARY OF THE INVENTION

The present invention provides a method for preparing an aqueoustreatment solution containing at least hydrogen peroxide ions andhydroxyl ions in predetermined concentrations, said predeterminedconcentrations being variable within a concentration range such thatsaid treatment solution has a total hydrogen peroxide concentration of0.1 to 10 grams/liter of solution, and a pH less than 14. The method ofthe invention comprises:

using at least one electrolytic cell comprising a porous cathode and ananode,

bringing into the proximity of said porous cathode, an aqueous alkalinesolution, the initial pH of which is chosen at a lower value than therequired value for the treatment solution to be prepared, said initialsolution constituting the catholyte,

circulating simultaneously, in contact with said porous cathode, anoxygen-containing compressed gas and said catholyte, so as to formhydrogen peroxide ions and hydroxyl ions at said cathode by electricalreduction of the oxygen on passing an electric current through saidcell,

continuing said circulation in contact with said porous cathode untilsaid catholyte attains said predetermined concentration of hydrogenperoxide ions and hydroxyl ions,

then finally discharging to the outside the catholyte which has attainedsaid predetermined concentration, the catholyte thus dischargedconstituting said treatment solution.

The present invention also provides an electrochemical reactor foreffecting said method.

One of the essential characteristics of the defined method is thus theutilization of the known principle of producing hydrogen peroxide byreducing oxygen at a cathode in order to directly prepare an aqueoustreatment solution having a low concentration of hydrogen peroxide ionsand hydroxyl ions (instead of utilising this principle to prepareproducts of high hydrogen peroxide concentration as in the known stateof the art). The direct electrolytic production of such a lowconcentration treatment solution is essentially possible because of theadvantageous use of an initial weakly concentrated alkaline solution asthe catholyte (the initial catholyte concentration being chosen at alower level than the required hydroxyl ion concentration for thetreatment solution), and by the correct choice of conditions under whichthe electrolysis is carried out, this electrolysis leading to aprogressively increasing concentration of hydrogen peroxide ions andhydroxyl ions in the catholyte, which is discharged to the outside whenthe required treatment solution concentrations are reached.

The parameters which govern the method according to the invention(initial concentration of alkaline solution, quantity of electricitypassing through the reactor, reactor dimensions etc.) are advantageouslyadjusted so that the solution discharged from the electrochemicalreactor has a total hydrogen peroxide concentration (i.e. hydrogenperoxide ions HO₂ ⁻ +hydrogen peroxide H₂ O₂) of 0.1 to 10 grams/literof solution, and a pH less than 14 (the aforesaid pH and hydrogenperoxide ranges correspond substantially to those encountered inconventional treatment solutions prepared by classical dilutionmethods). The alkaline solution fed initially into the electrochemicalreactor is chosen in particular such that its initial pH lies between 7and 12. The alkali which makes up this initial solution can be any knownalkali, such as caustic soda, potash etc. However, caustic soda isespecially used because of its low cost.

This direct electrolytic preparation of a treatment solution can becarried out by two different methods, namely a batch and a continuousmethod. The continuous method consists of operating with a catholytevolume which is chosen, depending on the cathode capacity, such that itcan be raised to the required concentration in a single passage acrossthe cathode, this method thus allowing continuous production of thetreatment solution by continuously feeding initial solution across thecathode. Such a method is particularly suitable for the continuousproduction of small quantities of treatment solution.

The batch method consists of operating with a catholyte volumesubstantially greater than the volume which could be processed in asingle passage by virtue of the cathode capacity, and of recycling thisvolume until it reaches the required concentration. Such a method isparticularly suitable for the batch production of large quantities oftreatment solution.

In addition, it can be envisaged to incorporate a certain number ofadditives in the initial solutions to be converted into bleachingsolutions by electrolysis, and in particular silicate buffers (dependingon the hydroxyl ion concentration) to keep the solution pH within arange of optimum values, preferably between 10.5 and 11, at thebeginning of the bleaching reaction.

The treatment solution obtained by the method according to the inventioncan in particular be used as a bleaching solution for bleachingmaterials such as textiles, pulp, paper pulp (cellulose pulp or highyield pulp), cardboard (surface bleaching), starch, bran etc. Thesematerials can be bleached either directly by contact or impregnation ifthe bleaching solution is prepared by a continuous method, or indirectlyby conventional systems such as bleaching towers (batch or semi-batch)if the bleaching solution is prepared by a batch method.

With regard to the bleaching of paper pulp, the constituents of thebleaching solution must notably satisfy precise concentration conditionsin order to give good bleaching performance. These concentrationconditions are generally expressed by paper manufacturing experts interms of the weight of pure constituents to be incorporated in a drypulp, and these then have to be recalculated (to express the quantity ofconstituent per unit of bleaching solution volume), taking into accountboth the initial consistency of the pulp to be bleached and the finaldensity of the pulp obtained after incorporating the bleaching solution.In this respect, the pulp to be bleached is generally never dry, butinstead has an initial greater or lesser consistency depending on thepretreatment given to the pulp, this initial consistency generally lyingbetween 20 and 88% (the consistency being given in times of theproportion of fibrous material in the wet pulp). Likewise, a pulp of adetermined consistency must generally be further diluted during thebleaching operation in order to facilitate treatment, this dilutionbeing carried out by adding the bleaching solution. The final density ofthe pulp obtained after adding this solution must generally be between 5and 18%.

Thus, in the case of a bleaching solution in which the alkali is causticsoda, it is generally assumed that the solution constituents must havethe following concentrations (referred to the dry pulp) to give correctbleaching: hydrogen peroxide 1 to 2 grams per 100 grams of dry pulp, andcaustic soda 0.5 to 2.5 grams per 100 grams of dry pulp. In order to beable to bleach paper pulp in every practical case (i.e bleaching a pulphaving an initial consistency before bleaching which can vary from 20 to88%, and a final density after adding the bleaching solution which canlie between 5 and 18%), the aforesaid conditions mean that it must bepossible to prepare a bleaching solution in which the hydrogen peroxideconcentration can vary from 0.4 to 9 grams per liter of bleachingsolution, and the caustic soda concentration can vary from 0.2 to 11grams per liter of bleaching solution, the respective proportions ofhydrogen peroxide and caustic soda in the solution being such that theweight ratio of the hydrogen peroxide to the caustic soda is between 0.4and 4 (as can be shown by relatively simple calculations). The methodaccording to the invention enables any bleaching solution within theseconcentration ranges to be prepared by suitably controlling the initialconcentration of the caustic soda solution used as the catholyte, andthe quantity of electricity fed through the electrochemical reactor.

The treatment solution obtained by the method according to the inventioncan be used not only for bleaching but for any other treatment whichutilizes the various properties of hydrogen peroxide, such as itsoxidation-reduction effect (chemical reactions for destroying bad odorsor an excess of strong oxidants such as chlorine), its bactericideeffect or its action as a source of dissolved oxygen. The treatmentsolution according to the invention can thus, by way of example, be usedfor the following: treatment of swimming pool water, treatment ofdomestic effluents, treatment of fish breeding water, addition of oxygenin dissolved form to living media requiring oxygen, etc.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing illustrates, diagrammatically by way ofexample, two embodiments and modifications of electrochemical reactorsfor effecting the method for preparing the treatment solution accordingto the invention.

FIG. 1 is a diagrammatic cross-section through a first embodiment.

FIG. 2 is a diagrammatic cross-section through a second embodiment.

FIG. 3 is a diagrammatic cross-section of a modification of FIG. 2,

FIG. 4 is a diagrammatic cross-section showing a first application ofthe reactor of FIG. 2.

FIG. 5 is a diagrammatic cross-section showing a second application ofthe reactor of FIG. 2.

FIG. 6 is a diagrammatic cross-section showing a third application ofthe reactor of FIG. 2.

SPECIFIC DESCRIPTION

The first embodiment shown in FIG. 1 is an electrochemical reactor ofthe "closed cell" type, which enables the treatment solution of theinvention to be prepared by a batch method. The reactor shown in FIG. 1comprises a single elementary cell 1 consisting of a substantiallyhorizontal casing 2 of an electrically insulating material. The casing 2is divided by a horizontal flat diaphragm 3 into two superposedcompartments 4 and 5, namely a lower cathode compartment 4 and an upperanode compartment 5. The cathode compartment 4 is itself divided by ahorizontal flat porous cathode 6 into two superposed chambers 7 and 8,namely an upper chamber 7 and a lower chamber 8. The upper chamber 7 isdesigned to contain the catholyte, and is provided at its opposing endswith an inlet nozzle 7a and an outlet nozzle 7b for the catholyte. Thepurpose of the lower chamber 8 is to feed compressed air (of the orderof 100 to 1000 grams/cm²) to the porous cathode 6 through an inletnozzle 8a connected to a source of compressed air (not shown), theexcess compressed air being able to escape through the safety valves 8b.The porous electrode 6 (gaseous diffusion electrode) is advantageouslyconstructed of a hydrophobic conducting material able to hold back thewater while allowing the air to pass. The porous cathode 6 can thus forexample be a carbon felt impregnated with a Teflon suspension, or porousblocks of vitreous carbon of open porosity, the thickness of thiscathode being advantageously between 5 and 10 mm.

The anode compartment 5 is designed to contain the anolyte, and isfitted with an anode 9 (or counter electrode) disposed against its upperwall, and an inlet nozzle 5a and outlet nozzle 5b for the anolyte, thesebeing disposed at its opposing ends. The anode 9 is advantageouslyconstructed of a conducting material which is chemically andelectrochemically inert towards the anolyte. The anode 9 can for examplebe a material such as platinum-plated titanium, graphite, suitablychosen stainless steel or any other material which constitutes adimensionally stable anode. The flat diaphragm 3 which separates theanolyte from the catholyte is advantageously in the form of a cationexchange membrane or a semi-permeable microporous diaphragm.

In the reactor of FIG. 1, the outlet nozzle 7b and inlet nozzle 7a ofthe chamber 7 are connected together by means for recycling thecatholyte, in the form of a conduit 10 and a pumping member 11.Likewise, the outlet nozzle 5b and inlet nozzle 5a of the anodecompartment 5 are connected together by means for recycling the anolyte,in the form of a conduit 12 and a pumping member 13. The reactor alsocomprises a first feed conduit 14 connected by way of a valve 15 to apart of the conduit 12 close to the inlet nozzle 5a of the anodecompartment 5, a second feed conduit 16 connected by way of a valve 17to a part of the conduit 10 close to the inlet nozzle 7a of the chamber7, and a discharge conduit 18 connected by way of a valve 19 to theconduit 10 upstream of the pumping member 11.

The first feed conduit 14 is used to feed into the anode compartment 5an initial caustic soda solution having an initial caustic sodaconcentration exceeding the required treatment solution concentration,this initial solution forming the anolyte for the reactor. The secondfeed conduit 16 is used to feed into the chamber 7 an initial causticsoda solution having an initial concentration less than the requiredtreatment solution concentration, this solution constituting thecatholyte for the reactor. The discharge conduit 18 enables thecatholyte to be discharged to the outside after it has been recycled acertain number of times to reach the required concentration, thedischarged catholyte then constituting the treatment solution.

The porous cathode 6 and anode 9 of the reactor are connected to thenegative and positive poles respectively of a direct current source 20.

The electrochemical reactor described operates in the following manner.The hydrogen peroxide and caustic soda concentrations in the treatmentsolution to be prepared are firstly chosen in accordance with therequired application (for example for bleaching a paper pulp of giveninitial consistency to within a final determined density), then the feedvalves 15 and 17 are opened, and a caustic soda solution of an initialconcentration exceeding the concentration of the treatment solution tobe prepared is fed through the feed conduit 14 into the anodecompartment 5, and a caustic soda solution of an initial concentrationless than the concentration of the treatment solution to be prepared isfed through the feed conduit 16 into the chamber 7 fitted with thecathode 6 (the fed quantities of solution are a function of therespective volumes of the chambers 5 and 7). The feed valves 15 and 17are closed, current is supplied to the electrochemical reactor, and themeans for feeding air to the cathode 6 and the means for recycling thecatholyte and anolyte are switched on. The reactions which then occur atthe electrodes are as follows.

At the anode:

    2OH.sup.- →1/2O.sub.2 +H.sub.2 O-2e.sup.-

At the cathode:

    O.sub.2 +H.sub.2 O+2e.sup.- →HO.sub.2.sup.- +OH.sup.-

It can thus be seen that when the catholyte passes through the chamber7, oxygen is reduced at the cathode to form hydrogen peroxide ions HO₂ ⁻and hydroxyl ions OH⁻, and as the catholyte continues to recycle, theconcentration of the hydrogen peroxide ions and hydroxyl ions in thecatholyte progressively increases. Likewise, as the anolyte passesthrough the anode compartment 5, the caustic soda concentration in theanolyte progressively falls. The catholyte and anolyte are recycleduntil the hydrogen peroxide ion and hydroxyl ion concentrations in thecatholyte reach the required values. When these values are reached,electrolysis is halted, and the catholyte is discharged to the outsidethrough the discharge conduit 18, the discharged catholyte forming therequired treatment solution to be used for any appropriate application.The anolyte, in which the caustic soda concentration has been greatlyreduced, is also discharged from the reactor. This discharged anolytecan be recovered to serve as the catholyte for preparing a new supply oftreatment solution (its transfer being indicated diagrammatically on thedrawing by the dashed line 21).

In the described embodiment, the reactor does not have to be necessarilyhorizontal. It can be particularly advantageous to dispose the reactorin a slightly inclined position (of the order of 10°), to prevent anyoxygen forming at the anode from accumulating in the anode compartment,and to thus facilitate its discharge to the outside.

In the electrochemical process described heretofore, it has been statedthat the cathode solution containing hydrogen peroxide constitutes theuseful product for direct use as the treatment solution in variousapplications, and the anode solution depleted in caustic sodaconstitutes only a by-product of little interest (except for the saidrecovery). However, it is possible to upvalue this by-productconstituting the anode solution by making small modifications to thepreviously described electrochemical process, to enable an anodesolution containing hypochlorite to be produced. To do this, it issimply necessary to feed into the anode compartment 5 an initial aqueoussolution containing both caustic soda in a higher concentration (about10 g/l) and sodium chloride (also about 10 g/l). By feeding this initialsolution containing sodium chloride into the anode compartment, it ispossible to feed an initial solution also containing sodium chlorideinto the cathode compartment, to improve the conductivity of thesolutions and reduce the ohmic drop through the cation exchangeseparator (this sodium chloride does not participate in the cathodereaction, but serves only as a supporting salt).

The purpose of the sodium chloride in solution in the anode compartmentis to cause formation of gaseous chlorine in contact with the anode, inaccordance with the reaction:

    2Cl.sup.- →Cl.sub.2 +2e.sup.-

The gaseous chlorine thus formed reacts with the caustic soda to givethe hypochlorite, by the reaction:

    Cl.sub.2 +2NaOH→NaoCl+NaCl·H.sub.2 O

As the anolyte is continuously recycled, its hypochlorite concentrationprogressively rises and its caustic soda concentration progressivelyfalls.

The anode solution obtained at the end of recycling, charged withhypochlorite, can then be used in combination with the cathode solutioncharged with hydrogen peroxide, for treating any suitable material. Thistreatment is advantageously effected by firstly bringing the anodesolution containing hypochlorite into contact with the material to betreated, then bringing the cathode solution containing hydrogen peroxideinto contact with this material. The combined use of hypochlorite andhydrogen peroxide has the great advantage of giving rise to theformation of "singlet" oxygen, which has proved to be an excellentoxidizing agent.

In the embodiment of the "closed cell" type heretofore described, themeans for recycling the catholyte and anolyte could be dispensed with,and the single reactor cell 1 could be replaced by a plurality of cellsdisposed in series one after the other, so as to give the requiredconcentration by a single passage through the various cells, thismodification then being allied to a continuous production method.

The second embodiment shown in FIG. 2 represents an electrochemicalreactor of "open cell" type, for preparing treatment solution accordingto the invention by a continuous method. The electrochemical reactorshown in FIG. 2 comprises a single elementary tubular cell 31. The cell31 is composed of two concentric porous tubular sheaths 32 and 33 withround bottoms, namely an outer sheath 32 constructed of an electricallyinsulating porous material such as porous ceramic or porouspolypropylene, and an inner sheath 33 constructed of a hydrophilicmicroporous insulating material such as cloth or felt (for example ofnylon, glass fiber or polyethylene). The open end of the sheaths 32 and33 is closed by a flat cover 34 of an insulating material. The innersheath 33 acts as a microporous diaphragm which divides the cell 31 intotwo coaxial compartments 35 and 36, namely a central cylindrical anodecompartment 35 and a peripheral annular cathode compartment 36. Thecentral anode compartment 35 is fitted with a porous tubular anode 37disposed against the inner wall of the microporous diaphragm 33, theporous anode 37 being constructed of a material such as porous graphiteor porous vitreous carbon. The anode compartment 35 is designed tocontain the anolyte, and is provided upperly with an inlet nozzle 38through the cover 34. The peripheral cathode compartment 36 is filledwith a tightly packed bed of electrically conducting particles 39 (suchas granules of graphite and active carbon, or carbon fibres), to serveas a three dimensional cathode. This bed is made partly hydrophobic bytreating it with a Teflon suspension. The bed of particles 39 istraversed over its entire length by a conduit 40 pierced with aplurality of apertures, its ends being connected respectively to aninlet nozzle 40a and an outlet nozzle 40b provided in two diametricallyopposing positions in the cover 34. About the conduit 40 there is wounda wire current collector 41 (for example of gold), for connection to thenegative pole of a direct current source 42, the positive pole beingconnected to the anode 37.

The perforated conduit 40 is connected by way of the inlet nozzle 40a toa source of compressed air 43 which can feed cold compressed air at apressure of the order of 20 to 50 g/cm², the purpose of the conduit 40therefore being to feed compressed air to the dispersed cathode 39 (theair being cold in order to maintain a temperature inside the cell 1 ofthe order of 15° to 20° C., so as to optimise the electrolysisefficiency). The inlet nozzle 38 of the anode compartment 35 isconnected via a pumping member 44 to a reservoir (not shown) containinga caustic soda solution having a concentration substantially identicalto (or slightly greater than) the caustic soda concentration of thetreatment solution to be prepared. The purpose of the pumping member 44is to continuously feed the anode compartment 35 with a caustic sodasolution under pressure, the pressure at which the caustic soda solutionis fed being chosen such that the pressure in the anode compartment 35is always greater than the pressure in the cathode compartment 36.

The described electrochemical reactor operates in the following manner:the hydrogen peroxide and caustic soda concentrations in the treatmentsolution to be prepared are chosen, current is fed to the cell 31, andthe means for feeding the caustic soda solution to the anode compartment35 and the means for feeding air to the dispersed cathode 39 arestarted, and the anode compartment 35 is constantly maintained at ahigher pressure than the cathode compartment 36. The caustic sodasolution which flows into the anode compartment 35 (forming the anolyteundergoes the anode reaction on contact with the anode 37 (this reactionis identical to that previously described), so that it becomes chargedwith oxygen while losing part of its caustic soda concentration. Thissolution, depleted in caustic soda, then passes through the microporousdiaphragm 33 because of the pressure difference. When it reaches thecathode compartment 36, the solution depleted in caustic soda (whichconstitutes the catholyte) then undergoes the cathode reaction (thisreaction being identical to that previously described), so that itbecomes charged in hydrogen peroxide ions and hydroxyl ions before beingdischarged to the outside in the form of droplets 45 on the outer wallof the porous ceramic 32. The parameters which govern the operation ofthe cell 31 (voltage and current density fed to the cell, pressure inthe cathode and anode compartments etc.) are chosen such that thecatholyte becomes enriched in hydrogen peroxide and hydroxyl ions to therequired concentration for the treatment solution just as the catholytereaches the exterior in the form of droplets 45, so that the collecteddroplets 45 form the treatment solution for use in the requiredapplication.

The reactor 51 shown in FIG. 3 constitutes a modification of the reactor31 of FIG. 2, and in which the positions of the cathode and anodecompartments are reversed relative to FIG. 2, while the other members ofthe reactor are substantially identical. The reactor 51 comprisesessentially two coaxial annular compartments 55 and 56 separated fromeach other by a microporous cylindrical diaphragm 53, namely an outeranode compartment 55 provided with an anode 57 and an inlet nozzle 58for the caustic soda solution, and an inner cathode compartment 56provided with a cathode 59 and inlet nozzle 60 for feeding oxygen. Theinner cylindrical wall 52 of the cathode compartment 56 is constitutedby a porous ceramic which allows the catholyte which has beenconcentrated in hydrogen peroxide ions and hydroxyl ions to pass, theconcentrated catholyte thus flowing to the outside in the form ofdroplets 45.

The treatment solution thus produced continuously in the form ofdroplets by the "open cell" reactors of FIGS. 2 and 3 can particularlyadvantageously be then used directly to carry out the required treatmentby the contact or impregnation method.

In this respect, FIG. 3 shows by way of example one possible applicationfor the reactor 51 illustrated in this Figure, and consisting of thedirect contact bleaching of material present in the form of pulp orfibre (such as paper pulp). For this application, the reactor 51 isfitted with an Archimedes screw 61 rotatably mounted in the tubularchannel 62 defined by the inner cylindrical wall 52. The material to bebleached is fed continuously to one end of the channel 62 (the feedbeing indicated diagrammatically by the arrow 63 on the drawing), and isthen progressively bleached in contact with the droplets 45 as it ismoved by the screw 61 along the channel 62 (material outlet showndiagrammatically on the drawing by the arrow 63).

FIG. 4 shows a first possible application of the reactor illustrated inFIG. 2, and consisting of bleaching material in the form of a continuousband 65 (such as fibrous sheet, foil), using a plurality of pairs ofreactors 31 mounted rotatably in the manner of rollers, between whichthe continuous band 65 moves.

FIG. 5 shows a second possible application for the reactor of FIG. 2,and consisting of treating liquid substances (such as regeneration ofswimming pool water) using a reactor 31 mounted inside a tubular conduit66 through which the liquid to be treated circulates (as a modification,an annular Archimedes screw could be mounted between the reactor 31 andconduit 66, to provide treatment similar to that shown in FIG. 3).

FIG. 6 shows a third possible application, consisting of regenerating aliquid medium using a reactor 31 inserted within a biological sandfilter 67 disposed on the bottom of a container 68 containing the liquidmedium 69 to be regenerated.

EXAMPLE 1

A cell for producing treatment solution analogous to FIG. 1 is used,having the following characteristics:

cathode of graphite felt made hydrophobic in depth by suitabletreatment;

anode of platinum plated titanium;

cation exchange membrane of the type sold commercially under the name ofNAFION 425 (by Messrs. Du Pont de Nemours). The area of the separator is2.85 dm².

An aqueous caustic solution having an initial caustic soda concentrationof 0.04 g/l is fed into the cathode compartment, and an aqueous causticsoda solution having an initial soda concentration of 1.1 g/l is fedinto the anode compartment, the solution volume fed into eachcompartment being 560 cm³ (the temperature of these solutions being keptat 22° C.).

Electricity is fed to the cell by connecting the terminals to a powersource, and the means for feeding air to the cathode and the means forrecycling the catholyte and anolyte are started. The cell is thenadjusted to deliver a final treatment solution (catholyte afterrecycling) containing 1.8 g/l of hydrogen peroxide and 2.88 g/l ofcaustic soda. To attain this, the voltage at the cathode is adjusted sothat it is equal to -950 mV relative to a Hg, HgO reference electrode(corresponding to a potential difference across the cell terminals of3.5 Volts and an average current density of 1 A/dm²), and the catholyteand anolyte are recycled for 50 minutes, after which the catholytereaches the required hydrogen peroxide and caustic soda concentrations(as stated heretofore). The catholyte is then discharged to the outside(555.5 cm³ are collected) for use as a treatment solution.

The final hydrogen peroxide Faraday efficiency is 66.4%, and the energyconsumption is 8.3 kwh per kg of 100% hydrogen peroxide.

The treatment solution thus obtained is used by way of example forbleaching mechanical wood pulp. This treatment solution (555.5 cm³) isthen immediately heated to 70° C., then intimately mixed with 444.5 g ofa mechanical wood pulp having an initial density of 22.5% (i.e. 100 g ofdry pulp) and an initial whiteness degree of 55. The final pulp densityobtained after this intimate mixing is 10% (the hydrogen peroxide andcaustic soda quantities with respect to the dry pulp then being 1% and1.6% respectively).

After mixing, the mixture is left for two hours while maintaining thepulp temperature at 70° C. The pH of the pulp which was initially 11.4then falls to 8.15 at the end of this two hour period, and the degree ofwhiteness rises from 55 to 63.

By way of comparison, an analogous bleaching operation is carried out bypurely chemical means using a synthesis treatment solution prepared fromcommercial hydrogen peroxide and caustic soda (the treatment beingidentical with regard to the batch of pulp used, the density of thepulp, the reagent concentration, the temperature and time). A degree ofwhiteness of 62.8 is obtained.

EXAMPLE 2

A cell for producing treatment solution practically identical to that ofexample 1 is used, except that a bed of active carbon granules isdisposed in the anode compartment (interposed between the compartmentwall and the current collector, which is constituted by a platinumplated titanium grid).

An aqueous solution of caustic soda and sodium chloride comprisinginitially 0.04 g/l of caustic soda and 10 g/l of sodium chloride is fedinto the cathode compartment of the cell, and an aqueous solution ofsoda and sodium chloride comprising initially 10 g/l of caustic soda and10 g/l of sodium chloride is fed into the anode compartment (thesolution volumes fed into each compartment being 650 cm³, thesesolutions being kept at 15° C.).

The cell is switched on, the cathode voltage is adjusted to -965 mVrelative to the Hg, HgO reference electrode (the potential differenceacross the cell terminals being then 2.2 V and the average currentdensity 3.5 A/dm²), and the catholyte and anolyte are recycled for 20minutes. After recycling, a solution is obtained in the cathodecompartment containing 2.2 g/l of hydrogen peroxide and 2.9 g/l ofcaustic soda, and a solution is obtained in the anode compartmentcontaining 1.6 g/l of hypochlorite. The final Faraday efficiency of thehydrogen peroxide production is 67%, and the energy consumption is 5.11kwh/kg of 100% hydrogen peroxide.

The solutions thus obtained are used for bleaching mechanical wood pulp.To carry out this bleaching, the following method can be used: 100 cm³of the anode solution containing the hypochlorite is taken and pouredinto 444.5 g of a pulp having an initial density of 22.5% (100 g of drypulp) and an initial degree of whiteness of 55. This pulp-solutionmixture is raised to 70° C. in 15 minutes while keeping the mixtureproperly mixed. 455.5 cm³ of the cathode solution containing thehydrogen peroxide are taken and added to the mixture. The mixture isintimately stirred, and then left to lie for 2 hours at 70° C. The finalmixture thus obtained consists of a pulp having a density of 10%(containing at the outset 1% of hydrogen peroxide with respect to thedry pulp).

The degree of whiteness measured after rinsing the pulp is 71 (initialdegree of whiteness 55).

We claim:
 1. An electrolytic oxidation and reduction method for preparing an aqueous treatment solution comprising hydrogen peroxide and hydroxyl ions in predetermined concentrations variable within a concentration range such that said treatment solution has a total hydrogen peroxide concentration of 0.1 to 10 g of H₂ O₂ /liter of solution and a predetermined pH of less than 14, which comprises the steps of:(a) applying an electric current across at least one flat, horizontal electrolytic cell comprising a horizontal anode in contact with an anolyte in a flat, horizontal anolyte compartment and a porous, horizontal cathode in contact with a catholyte in a flat, horizontal catholyte compartment, said anode and said cathode being separated from one another by a horizontal cationic exchange membrane or a horizontal semipermeable microporous diaphragm, said catholyte being constituted by an aqueous alkaline solution of an initial pH of 7 to 12 which is lower than the predetermined pH value of the treatment solution to be prepared, and wherein said anolyte compartment is located above said catholyte compartment; (b) circulating in contact with said porous cathode during application of said electric current, said catholyte, while feeding an oxygen-containing compressed gas through said cathode into said catholyte compartment to form hydrogen peroxide ions and hydroxyl ions at said cathode by electrolytic reduction of the oxygen; (c) continuing said catholyte circulation in contact with said porous cathode, without otherwise altering its chemical composition until said catholyte attains said predetermined concentration of hydrogen peroxide ions and hydroxyl ions to form an aqueous treatment solution; (d) following step (c), terminating said circulation and cutting off said electric current; and (e) discharging the electrolyte as said aqueous treatment solution with a weight ratio of hydrogen peroxide to sodium hydroxide between 0.4 and
 4. 